z

class User < ActiveRecord::Base

 def key_is_valid?
  ...
  end
end

# A "helper"
def user_key_is_valid?(user, params)
  user.user_key == params[:user_key]
end

# Imagine that we're here inside of a controller...
@user.user_key = params[:user_key]
@user.key_is_valid?(params[:user_key])
user_key_is_valid?(User.find(params[:id]), params)